Aromatic alkylation process

ABSTRACT

A process for the catalytic distillation production of alkylated aromatic compounds is provided wherein the vapor pressure of the olefin may be increased while maintaining the same olefin feed rate and aromatic to olefin ratio. In one embodiment a side stream from the vapor from the second column below the catalyst and olefin feed is condensed and rerouted to the aromatic make up stream from the reflux drum. The vapor pressure of the olefin in the lower end of the first column in the catalyst bed is thus increased which increases the equilibrium concentration of the olefin in the liquid phase. In another embodiment of the invention the effective driving force for the reaction is increased by injecting the olefin at different heights within the catalyst bed. If additional olefin is injected more catalyst bed height would be required, but the additional catalyst is more that offset by the increased throughput at the same overall olefin conversion. The practice of injecting the olefin feed at different heights is especially useful when the same olefin feed rate is used but split among the several streams because less catalyst is required.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a catalytic distillation process forthe alkylation of organic aromatic compounds with olefins over an acidiccatalytic distillation structure. More particularly the inventionrelates to an improvement in the process whereby the olefinconcentration in the liquid phase is increased by increasing the olefinpartial pressure in the vapor phase while maintaining a constant olefinfeed rate and benzene to olefin ratio.

2. Related Art

Alkylation of organic aromatic compounds using catalytic distillationhas been disclosed in U.S. Pat. No. 4,849,569 issued to Lawrence A.Smith, Jr. As practiced the process disclosed therein is embodied by twoseparate columns connected by liquid and vapor flow lines with onecolumn being filled with a bed of the catalytic distillation structureand the second containing standard distillation structure. The olefin isfed below the catalyst bed, usually into the top of the second column.The aromatic compound is fed with the reflux into the top of the firstcolumn above the catalyst bed.

Theoretically the two columns should act as one continuous column.Increasing the olefin concentration in the vapor will increase theequilibrium olefin concentration in the liquid phase and thus thedriving force for the reaction; resulting in a more economical process.However, the effect on the projected catalyst life may be deleterious asit has been found that a key variable in catalyst aging is theconcentration of the olefin in the liquid phase in contact with thecatalyst. Another associated effect is a decrease in the liquid benzeneloading throughout the system which detrimentally affects the reactionkinetics and selectivities due to the reduction in the critical benzeneto olefin ratio.

SUMMARY OF THE INVENTION

Briefly the invention is an improvement of the process for thealkylation of organic aromatic compounds which utilizes catalyticdistillation wherein the improvement comprises the ability to increasethe olefin vapor pressure in the catalyst bed while maintaining theolefin feed rate and the olefin to aromatic ratio. In one embodiment aside stream from the vapor from the second column below the catalyst andolefin feed is condensed and rerouted to the aromatic make up streamfrom the reflux drum. The vapor pressure of the olefin in the lower endof the first column in the catalyst bed is thus increased whichincreases the equilibrium concentration of the olefin in the liquidphase. In a similar embodiment a refluxed enriching section is added tothe second column above the liquid feed from the first column whichimproves the vapor quality.

In another embodiment of the invention the effective driving force forthe reaction is increased by injecting the olefin at different heightswithin the catalyst bed. If additional olefin is injected more catalystbed height would be required, but the additional catalyst is more thanoffset by the increased throughput at the same overall olefinconversion. However, the practice of injecting the olefin feed atdifferent heights is especially useful when the same olefin feed rate isused but split among the several streams because less catalyst, hence asmaller reactor is required for the same alkylation throughput. dr

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a simplified flow diagram in schematic form as the process wasformerly practiced.

FIG. 2 is a simplified flow diagram in schematic form showing oneembodiment of the present invention.

FIG. 3 is a simplified flow diagram in schematic form showing a secondembodiment of the present invention.

FIG. 4 is a simplified flow diagram in schematic form showing a thirdembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For an understanding of the present invention the reader should first befamiliar with previous design practices and operation. For a detaileddescription of the general principles associated with the alkylation oforganic aromatic compounds utilizing the catalytic distillation process,the reader is referred to the above mentioned U.S. Pat. No. 4,849,569.

FIG. 1 shows in a very simplified form the previous design practice fororganic aromatic alkylation as embodied in the production ofethylbenzene from the reaction of benzene with ethylene. Other organicaromatic compounds and olefins may be used as feeds yielding differentproducts or product mixes. Additionally, polysubstituted aromaticcompounds may also be produced and separated for transalkylation.

The previous operation utilizes a distillation column reactor having an"upper" column lo containing the particulate acid catalyst 12 in theform of catalytic distillation structures. This "upper" column iscommonly referred to as the distillation reaction zone. A "lower" column14 contains standard distillation structure and is referred to as thebenzene stripper which completes the separation of unreacted benzenefrom the higher boiling reaction products--ethylbenzene andpolysubstituted aromatic compounds. In this operation the olefin is fedvia flow line 1 to the top of the benzene stripper 14 and make upbenzene is fed to the reflux drum 18 via flow line 9. Essentially all ofthe olefin is converted in the distillation reaction zone 12 so thatonly benzene and any inert lights are taken overhead via flow line 3,condensed in partial condenser 16 and separated in reflux drum 18. Theuncondensed light inerts are removed via flow line 7.

The bottoms from the "upper" column 12, containing the higher boilingreaction products and some unreacted benzene are fed via line 13 to theupper end of the benzene stripper 14 where the reaction products,predominantly ethylbenzene, are removed as bottoms via line 17 forfurther processing if desired.

In the previous design the upper column 10 containing the distillationreaction zone behaves as a packed absorber column. For a fixed volume ofcatalyst, the olefin absorption rate into the liquid phase is limited bythe average partial pressure of olefin in the vapor phase as it travelsupward through the distillation reaction zone 12. As the effectivevolume of catalyst decreases due to aging, higher average olefin partialpressure is required to maintain the minimum olefin conversion.

The process embodied in the present invention provides the capability toraise this driving force and allows variation of the driving force atwill during the life of the catalyst.

In a first embodiment as shown in FIG. 2 a condenser 20 is provided tocondense a slip stream 19 of the vapor from the benzene stripper 14. Theolefin feed line 1 is moved to the bottom of the "upper" column 10 toprevent any of the olefin from being withdrawn with the slip stream. Theolefin feed line 1 could be injected into the vapor return line 15upstream of the slip stream draw off. The slip stream 19 lowers theconcentration of benzene in the vapor and thus increases the olefinconcentration at the lower end of the distillation reaction zone 12. Thecondensed liquid from the condenser 20 is routed via line 21 to the topof the distillation reaction zone 12 by combining it with the reflux inline 11. Thus the critical benzene to olefin ratio in the distillationreaction zone 12 is maintained. Alternatively, the condenser 20 may beused as a knock back condenser with the condensed liquid being returnedto the top of the benzene stripper or used elsewhere in the process andthe reduced vapor flowing on to the "upper" column via flow line 15.

A second embodiment is shown in FIG. 3. A Short enriching section 14A isadded above the liquid inlet to the benzene stripper to provide moreefficient separation in the stripper 14. If desired, the enrichingsection 14A may include a reflux via line 23 as shown. In either case areduced vapor enriched in benzene is provided to the bottom of thedistillation reaction zone.

In a third embodiment shown in FIG. 4 the olefin partial pressure iseffectively increased by dividing the olefin feed in to several streamsand feeding at different heights in the distillation reaction zone 12.Although there is no actual increase in the olefin this improvementextends the cycle time with a resultant increase in olefin in the liquidphase.

Under the process as described by FIG. 1 the partial pressure of theolefin in the vapor phase is highest at the bottom and lowest at the topdue to the progressive reaction up through the reaction zone bed 12. Theequilibrium concentration in the liquid phase follows the same profile.In a pilot unit demonstration plant having 29 feet of catalyst height inthe distillation reaction zone, the overall conversions of olefin at sixfeet and twelve feet above the bottom of the bed are about 40 and 63%respectively. These conversions represent a driving force reduction ofabout 40% per six feet of bed.

If additional olefin is fed instead of simply dividing the same feedstream, it should be appreciated that additional bed height would benecessary to ensure that the overall conversion of the combined olefinfeed remains at an acceptable level. However, an unexpected result isthat the extra throughput of olefin obtainable at the same overallconversion is beyond that which would be expected by the simple additionof olefin and catalyst height. At a constant olefin feed rate thisconverts into less catalyst flow area needed for the same height ofcatalyst and conversion level. For simplicity the following cases arepresented for double olefin feed points, but can be applied to anynumber of feed points limited only by practical considerations.

Case 1

An additional six foot catalyst bed is installed above the top bed ofthe original five bed reaction distillation zone (30 feet original bedheight). Additional olefin equal to about 40% of the original amount fedat the bottom of the zone is fed between the bottommost bed and the nexthigher. All of the olefins are in contact with at least 30 feet ofcatalyst and the overall conversion of the combined olefin feed is morethan maintained. The advantage that is realized is that a 40% increasein olefin throughput can be achieved with only a 20% increase inadditional bed height. At a constant throughput this represents areduction in the cross sectional area of the distillation reaction zoneof about 30% and a net reduction in the required catalyst volume ofabout 20%.

Case 2

Two additional six foot beds are added to the standard five bedarrangement for a total bed height of 42 feet. Additional olefin of 63%of the original amount is fed between the second and third beds from thebottom. Again all of the olefin contacts at least 30 feet of catalyst toachieve the same overall conversion rate. The advantage thus gained isthat a 63% increase in throughput is achieved with only a 40% increasein bed height. At constant throughput this represents a reduction incross section area of 40% and a 20% reduction in catalyst volumerequirement.

The invention claimed is:
 1. In a catalytic distillation process for thealkylation of organic aromatic compounds with olefins over a particulateacidic catalytic distillation structure comprising the steps of feedinga stream containing olefins and a stream containing organic aromaticcompounds to a distillation column reactor and concurrently reacting theolefins and organic aromatic compounds and separating the resultingreaction product by fractional distillation, the improvement comprisingthe increasing the olefin partial pressure through out the catalyst bedwhile maintaining a constant olefin feed rate and maintaining a constantaromatic compound to olefin ratio, wherein the effective olefin partialpressure is increased by dividing the olefin feed into at least twoseparate streams and feeding said separate streams at differentlocations along said catalyst bed.
 2. A process for the alkylation oforganic aromatic compounds with olefins, comprising the steps of:(a)dividing a first stream containing olefins into a second and thirdstreams of the same composition and feeding said second and thirdstreams to a distillation column reactor at different heights in adistillation reaction zone and feeding a fourth stream containingorganic aromatic compounds to said distillation column reactor near thetop of said distillation reaction; (b) contacting said olefins and saidorganic aromatic compounds together with a particulate acidic catalyticdistillation structure in said distillation reaction zone therebyreacting substantially all of said olefins with a portion of saidorganic aromatic compounds to form a reaction mixture containingalkylated aromatic product and unreacted organic aromatic compoundswhile distilling a portion of said unreacted organic aromatic compoundsoverhead of said distillation column reactor and distilling saidalkylated aromatic product and the remainder of said unreacted organicaromatic compounds downward out of said distillation reaction zone; and(c) separating said alkylated product from said remainder of unreactedorganic aromatic compounds by fractional distillation in a separatedistillation zone, said remainder of unreacted organic aromaticcompounds being recovered as overheads from said distillation zone andsaid alkylated product being recovered as bottoms from said distillationzone.